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REPRESENTATIVE MICHAEL Capuano’s announcement that he will not seek the Democratic nomination to challenge Senator Scott Brown suggests that, barring a surprise, the field for next year’s US Senate election is set. Consumer advocate Elizabeth Warren has yet to make an announcement, but that’s increasingly a formality as she breezes through a carefully scripted roll-out, most recently delivering a rousing keynote speech at a union-sponsored Labor Day breakfast.

With Capuano on the sidelines, the Democrats will be challenging Brown with a field of relative newcomers and outsiders. They include Warren, the Harvard law professor who shot to prominence in establishing the new consumer-protection bureau under the Dodd-Frank law; Alan Khazei, the co-founder of City Year and leading advocate for national service programs; Bob Massie, the environmentalist and one-time Democratic nominee for lieutenant governor; and Newton Mayor Setti Warren, an Iraq war vet whose door-knocking campaign led to his surprise victory in 2009.

None of the Democrats has spent much time in office, but this rich and varied field offers something different: Each fits the citizen-politician model that voters embraced in Deval Patrick in 2006 and Brown himself in last year’s special Senate election. Brown, in fact, still carries some of that Mr. Smith innocence; a year and a half after upsetting Attorney General Martha Coakley, he’s still learning the ropes. While his political personality is well developed, his agenda is not.

He ran to be the 41st vote necessary to block President Obama’s health care overhaul, and forced Obama to go through many uncomfortable gyrations to get around Senate opposition. But even though he’s no longer the 41st vote, Brown remains a pivotal figure as one of the relatively few Republicans not in lockstep with GOP Senate leader Mitch McConnell. His independence is admirable, but he has yet to turn it into a source of power. He’s often among the last to make up his mind about his colleagues’ initiatives, while pursuing relatively few of his own. Brown will have another year to prove that his celebrity status in Washington can produce real clout for Massachusetts. He must help advance the state’s priorities in medical research and the development of renewable energy, among other bipartisan causes.

But while Brown is still unformed, so too are the Democrats. As with Brown, their personas are clearer than the approaches they might take in office. Each can present himself or herself as a more liberal alternative to Brown - as someone who’s better aligned with the rest of the state’s congressional delegation. It should also be easy for Brown to counter them with his everyman affability and soft ideological edges. But with a year to go until the primary, voters should expect more than the usual ideological positioning; both Brown and his challengers should be called upon to deliver fresh, thoughtful assessments of what ails the state and the nation.

Massachusetts has a robust history of expecting its senators to be national leaders. It should demand nothing less of Brown and the Democratic newcomers who are lining up to run against him.